1. Field of the Invention
The present invention relates generally to the fields of de-watering sludge. More particularly, it concerns methods and apparatus for de-watering sludge by utilizing a multi-stage, rotating drum technique.
2. Description of Related Art
The ability to remove water from products such as waste products has long been recognized as useful. Removing water from sludge, and more particularly, sewage sludge, greatly reduces the weight of material to be transported for disposal and/or use. Additionally, de-watering sludge may facilitate the processing of sludge into soil conditioners and fertilizers.
De-watering of sludge has been accomplished in the past by various methods, including open-air drying, vacuum filtration, centrifugation, mechanical pressing, and other mechanical separation. Although each has utilized at least a degree of usefulness, room for significant improvement remains.
U.S. Pat. No. 3,695,173 describes a technique for sludge de-watering. There, de-watering is achieved as sludge is moved through a filter de-watering unit, where squeezing and pressing takes place. The squeezing and pressing forces most of the liquid or water through a filtering medium or slots. Cleaning blades or brushes are used prevent clogging and interruption of continuous filtration or de-watering. Although this technique may be useful for de-watering certain types of sludge, problems remain. For instance, if there is too little sludge being fed into the device, the sludge may not be squeezed enough. Consequently it may not be dried sufficiently. Further, if too much sludge enters the device, it may come under too much pressure and may be forced out through the device. Consequently, sludge may be lost and may need to be recirculated. Still further, if a polymer has been used on the sludge being de-watered, this system may, if subjected to high pressures, break polymer bonds and may therefore not be effective in de-watering sewage sludge, especially re-circulated sludge.
U.S. Pat. No. 4,286,512 describes another technique that may be used to remove liquid from a material. There, a screw press is used for pressing liquid from fibrous slurries, such as paper pulp, sludge, or sedimentation. Material is fed into the press at one end of a rotating press screw. The core of the screw has an increasing diameter such that the space defined between the core and wall of a drum gradually decreases so as to aid in the pressing process. A rotating drum rotates about the core to further aid in the draining of liquid pressed from the slurry due to the action of the press-screw. Although this technique also may be useful for de-watering certain types of sludge, similar problems remain due mostly to the fact that this technique relies upon pressing of the sludge to achieve drying. Again, if there is too little sludge being fed into the device, the sludge may not be squeezed enough. On the other hand, if too much sludge enters the device, it may come under too much pressure and may be prematurely forced out of the device. Consequently, sludge may be lost and may need to be recirculated. Still further, if a polymer has been used on the sludge being de-watered, this system may, if subjected to high pressures, break polymer bonds. For this reason, such a system may not be effective in de-watering sewage sludge, especially recirculated sludge.
U.S. Pat. No. 4,202,773 describes another technique that may be used to de-water sludge. There, clarifier sludge may be de-watered by passing it through an inner centrifuge drum and then to outer second drum having a diameter about twice that of the first drum. In each drum, wipers or scrapers displace solids. Each drum is unperforated, the outer drum is conical over its entire length, and the inner drum is conical in part or in whole. Although this system may be useful for de-watering certain materials, problems remain. For instance, it appears that sludge is spun at relatively high speeds to insure that solids within the sludge are displaced and so that sludge does not overly mix with water and liquids being extracted. Further, it appears that this device may subject sludge to great forces, particularly after transfer to the larger outer centrifuge, so that polymer bonds may be broken.
Problems enumerated above are not intended to be exhaustive but rather are among several that tend to impair the effectiveness of previously known devices for removing water and liquids from sludge. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that devices appearing in the art have not been altogether satisfactory.
In one respect, the invention is an apparatus for de-watering sludge, including an outer housing, a main shaft, a screw shaft, a first stage drum, a second stage drum, an inlet, and screw flighting. The outer housing defines a longitudinal axis. The main shaft is configured to rotate about the longitudinal axis. The screw shaft is coupled to the main shaft and is configured to rotate about the longitudinal axis. The first stage drum is positioned between the outer housing and the screw shaft. The first stage drum is configured to rotate about the longitudinal axis, and the first stage drum has a first drum diameter. The second stage drum is coupled to the first stage drum. The second stage drum is configured to rotate about the longitudinal axis, and the second stage drum has a second drum diameter greater than the first drum diameter. The inlet is configured to introduce the sludge to a first area defined by an outer surface of the screw shaft and an inner surface of the first stage drum. The screw flighting is coupled to the screw shaft. The screw flighting is configured to rotate about the longitudinal axis to transport the sludge longitudinally from the first area to a second area defined by an outer surface of the screw shaft and an inner surface of the second stage drum, the second area being larger than the first area.
In other respects, the main shaft and the screw shaft may each be configured to rotate at a first rate, and the first stage drum and the second stage drum may each be configured to rotate at a second rate. The first rate may be less than the second rate. The first rate may equal the second rate. The main shaft may have a substantially constant outer diameter, and the screw shaft may have a substantially constant outer diameter. The apparatus may also include wiper coupled to the screw flighting. The wiper may be configured to clean an inner surface of the first stage drum and to roll the sludge from the first area to the second area. The first stage drum may include a first slot, and the second stage drum may include a second slot. Each of the first and second slots may include a smaller opening on an inner surface of the drums than on an outer surface of the drums. The first slot may be smaller than the second slot. The first slot may be about the same size as the second slot. The apparatus may also include a third stage drum. The third stage drum may be coupled to the second stage drum. The third stage drum may be configured to rotate about the longitudinal axis. The third stage drum may have a third drum diameter greater than the second drum diameter. The screw flighting may be configured to transport the sludge longitudinally from the second area to a third area defined by an outer surface of the screw shaft and an inner surface of the third stage drum, the third area being larger than the second area. The apparatus may also include a fourth stage drum. The fourth stage drum may be coupled to the third stage drum. The fourth stage drum may be configured to rotate about the longitudinal axis. The fourth stage drum may have a fourth drum diameter greater than the third drum diameter. The screw flighting may be configured to transport the sludge longitudinally from the third area to a fourth area defined by an outer surface of the screw shaft and an inner surface of the fourth stage drum, the fourth area being larger than the third area. The apparatus may also include a wash nozzle coupled to the screw shaft. The outer housing may be inclined.
In another respect, the invention is an apparatus for de-watering sludge, including an outer housing, a main shaft, a screw shaft, a drum, an inlet, screw flighting, and a wiper. The outer housing defines a longitudinal axis. The main shaft is configured to rotate about the longitudinal axis. The screw shaft is coupled to the main shaft and is configured to rotate about the longitudinal axis. The drum is positioned between the outer housing and the screw shaft and is configured to rotate about the longitudinal axis. The drum has a slot including a smaller opening on an inner surface of the drum than on an outer surface of the drum. The inlet is configured to introduce the sludge to an area defined by an outer surface of the screw shaft and an inner surface of the drum. The screw flighting is coupled to the screw shaft and is configured to rotate about the longitudinal axis to transport the sludge longitudinally along the drum. The wiper is coupled to the screw flighting and is configured to clean an inner surface of the drum and to roll the sludge along the drum.
In another respect, the invention is an apparatus for de-watering sludge including an outer housing, a main shaft, a screw shaft, a wash nozzle, a first stage drum, a second stage drum, an inlet, a screw flighting, and a wiper. The outer housing defines a longitudinal axis. The main shaft is configured to rotate about the longitudinal axis. The screw shaft is coupled to the main shaft and is configured to rotate about the longitudinal axis. The wash nozzle is coupled to the screw shaft. The first stage drum is positioned between the outer housing and the screw shaft. The first stage drum is configured to rotate about the longitudinal axis, and the first stage drum has a first drum diameter and a first slot. The first slot includes a smaller opening on an inner surface of the first stage drum than on an outer surface of the first stage drum. The second stage drum is coupled to the first stage drum. The second stage drum is configured to rotate about the longitudinal axis, and the second stage drum has a second drum diameter greater than the first drum diameter and a second slot. The second slot includes a smaller opening on an inner surface of the second stage drum than on an outer surface of the second stage drum. The inlet is configured to introduce the sludge to a first area defined by an outer surface of the screw shaft and an inner surface of the first stage drum. The screw flighting is coupled to the screw shaft. The screw flighting is configured to rotate about the longitudinal axis to transport the sludge longitudinally from the first area to a second area defined by an outer surface of the screw shaft and an inner surface of the second stage drum, the second area being larger than the first area. The wiper is coupled to the screw flighting and is configured to clean an inner surface of the first stage drum and to roll the sludge from the first area to the second area.
In other respects, the main shaft and the screw shaft may each be configured to rotate at a first rate, and the first stage drum and the second stage drum may each be configured to rotate at a second rate. The first rate may be less than the second rate. The first rate may equal the second rate.
In another respect, the invention is a method for de-watering sludge. A main shaft is rotated about a longitudinal axis at a first rate. A screw shaft coupled to the main shaft is rotated about the longitudinal axis at the first rate. Screw flighting coupled to the screw shaft is rotated about the longitudinal axis at the first rate. A first and second stage drum are rotated about the longitudinal axis at a second rate. The first and second stage drums are positioned between an outer housing and the screw shaft. Sludge is introduced to a first area defined by an outer surface of the screw shaft and an inner surface of the first stage drum. Moisture is removed from the sludge through a first slot coupled to the first stage drum. The sludge is transported with the screw flighting from the first area to a second area defined by an outer surface of the screw shaft and an inner surface of the second stage drum, the second area being larger than the first area. Moisture is removed from the sludge through a second slot coupled to the second stage drum.
In other respects, the first rate may be less than the second rate. The first rate may equal the second rate. The method may also include cleaning an inner surface of the first stage drum with a wiper coupled to the screw flighting. The method may also include cleaning an inner surface of the first stage drum with a wash nozzle coupled to the screw shaft. The method may also include rotating a third stage drum about the longitudinal axis at the second rate, the third stage drum being positioned between an outer housing and the screw shaft. The sludge may be transported with the screw flighting from the second area to a third area defined by an outer surface of the screw shaft and an inner surface of the third stage drum, the third area being larger than the second area. Moisture may be removed from the sludge through a third slot coupled to the third so drum. The method may also include rotating a fourth stage drum about the longitudinal axis at the second rate, the fourth stage drum being positioned between an outer housing and the screw shaft. The sludge may be transported with the screw flighting from the third area to a fourth area defined by an outer surface of the screw shaft and an inner surface of the fourth stage drum, the fourth area being larger than the third area. Moisture may be removed from the sludge through a fourth slot coupled to the fourth stage drum.